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UNDERSTANDING REPETITION IN NATURAL LANGUAGE
INSTRUCTIONS - THE SEMANTICS OF EXTENT
Sheila
Rock
Department of Artificial Intelligence,Edinburgh University*
80 South Bridge, Edinburgh EH1 1HN, Scotland, United Kingdom
sheilaraisb.ed.ac.uk
Introduction
Natural language instructions, though prevalent in
many spheres of communication, have only recently
begun to receive attention within computational
linguistics[5]. Instructions are often accompanied
by language intended to signal repetition of the ac-
tion that they instruct. In order to develop a sys-
tem that is able to understand instructions, with
the goal of executing them, it is necessary to inves-
tigate what is meant by various types of repetition,
and the different ways in which repetition can be
expressed.
We focus on sentences that are instructing that
some action is to be performed and that this action
is to be performed more than once 1. There are two
aspects to consider - scope (what part of the action
that is instructed in the dialogue is to be repeated)
and extent (how much repetition is to be done).
This is illustrated by examples (1) and (2).
Place a chunk of rhubarb into each tart. (1)
Continue to layer in this way until all the (2)
fruit is used.
The repetition in (1) has scope on place a chunk
of rhubarb (into a tart) and extent across all tarts.

(2) has scope over layer in this way and extent
until
all the fruit used. Within this framework of scope
and extent that I have described only informally, I
discuss the issue of extent in more detail s .
Karlin [3], presents a semantic analysis of verbal
modifiers in the domain of cooking tasks. Much of
this is pertinent to an examination of extent, in par-
tieular the relation of different modifiers to the as-
peetual category of an event (according to Moens &
Steedman [4]). This has formed an important start-
ing point for my work in understanding instructions
for repetition. However, there are aspects where a
different approach to Karlin's is required, and some
of these are discussed in the rest of this paper.
Semantics of verbal modifiers
In analysing the semantics of verbal modifiers,
Karlin[3] identifies three kinds of modifiers, which
are themselves divided further. The primary cate-
gorisations are
*Thanks to Chris Mellish, Robert Dale and Graeme Ritchie
for discussion about the ideas in this paper.
1 This paper deals only with instructions, and uses the words
sentence and instruction interchangeably.
2A central theme of my thesis is that both scope and extent
must be accounted for in a full treatment of repetition, but a
discussion of scope is outwith the scope of this paper.
279
1 The number of repetitions of an action.
2 The duration of an action.

3 The speed of an action.
It is clear that Karlin's first two primary cate-
gories describe modifiers that are concerned with
the repetition of an action 3, and these are exam-
ined in detail in the next sections. First, though, it
is useful to consider that with any action, we have
a time interval, during which the action is to be
performed - once or more than once. We can then
characterise the extent of repetition in terms of this
time interval. Modifiers of Karlin's category 2 tell
us how long the time interval is, while modifiers of
category 1 tell us how to carve up the time interval.
One instruction may give information for both cat-
egories, but this usually is for two different actions,
such as
Roast
for 45 minutes, basting twice. (3)
Number of repetitions - carving the
interval
In this category, Karlin enumerates classes of mod-
ifier as follows:
• cardinal count adverbials - turn the fish twice
• frequency adverbials - turn the fish occasionally
• plural objects - turn the pieces offish
In the discussion of frequency adverbials, Karlin
describes frequency as a continuous scale with grad-
able terms, such as occasionally, often. This class
should include explicit frequency in time units, as
in every 5 minutes.
Duration of an action - delimiting the

interval
Here, Karlin enumerates the following kinds of
modifier:
• explicit duration in time intervals - fry the fish
for 10 minutes
• duration given by gradable terms - fry the fish
brie/Ty
• duration co-extensive with the duration of an-
other action - continue to fry the millet, stirring,
until the water boils
• duration characterized by a st'~te change - fry the
fish until it is opaque
• disjuncts of explicit durations and state changes
- fry the fish for 5 minutes or until it becomes
opaque
3I will not consider the third, which contributes to "quality"
of execution of an action, and does not pertain to extent of
repetition.
In this category, Karlin does distinguish be-
tween "explicit duration" and "duration in grad-
able terms", whereas in the 'Trequency adverbials"
classification, there are not seperate classes for
vague and explicit frequency (say turn the fish ev-
ery 5 minutes and turn the fish often). To be more
consistent, there should be one class within the cat-
egory "number of repetitions of an action" that
contains frequency adverbials 4, and only one class
within the cat~gory "duration of an action" that
contains duration in terms of time 5. In both classes
there should be the possibility of being explicit or

vague. It is also preferable to call Karlin's second
category "duration of repetition of an action". The
name "duration of an action" conflates the concept
of the basic action and its repetition. The sepa-
ration is pertinent to the view that repetition has
scope and extent.
Karlin analyses the remaining three classes in cat-
egory 2 explicitly in the context of cooking tasks.
In particular, the analysis is related to the view
that all processes in the cooking domain must have
culminations. The validity of this approach is dis-
cussed in the next section. However, before doing
that we examine Karlin's final class, "disjuncts of
explicit durations and state changes". This is a
class of instructions found mainly in the cooking
domain. The example used by Karlin is (4).
Steam for g minutes or until the mussels (4)
open.
Karlin asserts that 'the meaning of sentences in
this category is not the same as that of logical
disjunction'[3, pg 64], and claims that the mean-
ing of the disjunction is that 'the state change
(the mussels are open) is to be used to determine
the duration of the action (2 minutes)' [ibid] (my
parentheses) s .
I agree that the meaning is not simply that of log-
ical disjunction, but we need to examine the issue
further. Data that I have collected gives evidence
that the use of the or is not significant. There are
many examples where a recipe book will give the

same instruction, both with and without it. For
example,
at least 10 minutes or until the flour is (5)
well browned [2, pg 120]
Bake for
about
g hours, until the rabbit and
lentils are tender [2, pgll9] (6)
Bake for 45 minutes or until the rabbit is (7)
tender [2, pgll8]
In all of these, we have an instruction describing
one of the following scenariosT:
Do some action until an expected state
change occurs. This should take the du- (8)
ration specified.
4This is as Karlin's classification
5This is different from Karlin's classification
6Karlin sees these as metalinguistic disjunction, which I be-
lieve is similar to part of my view.
7I make no claims about exactly which of these scenarios is
being described.
280
Do some action for a specified duration. If
the expected state change does not occur
during this time, then it is likely that some- (9)
thing has gone wrong.
What is really being given is a way to decide when
to stop the action, and the use of two clauses pro-
vides a way of deciding whether the stop state is
successful or a failure. For success, if the state

change has occured, then we will expect that the
duration has also passed s. If the duration has
passed but the state change has not oecured, or if
the state change has occurred but the duration has
not passed, we still reach the stop state, but in the
failed mode. We then have disjunction for stopping
(we stop if either the duration or the state change is
true) but conjunction for success (stop and a nor-
mal outcome is only true if both the clauses are
true). We note that often domain knowledge will
allow the hearer to determine whether the duration
is given as a minimum or maximum time, and what
the effect of failure is. The analysis presented here
does not take the use of domain knowledge into ac-
count, to give a more general analysis.
From the point of view of repetition, what we
are given is a stopping condition, that is coded in
terms of two alternatives. Using an informal no-
tation, what is being expressed with and without
or respectively, are the following, which are equiv-
alent:
should-stop(action, t)*
(difference(start, t,x), x >_ duration) V (state(q,t))
should-stop(action, t)*- (difference(start, t,x),
x >_ duration) A
should-stop(action, t), (state(q,t))
Thus (7) artd (6) can be represented as
should-stop(bake, t)~
(differenee(start, t,x), x >45-minutes )
V (tender(rabbit, t))

should-stop(bake, t), (difference(start, t,z),
av >_ about.2-hours) A
shoutd-stop(bake, O, (tender(rabbit-and-lentils, t))
Sometimes, the order of the two modifiers is
different 9 indicating that the positioning of the
clauses is not important.
until the meat is tender, about 45 min'llO~t )
utes
[2, pg 119]
until the meat is meltingly tender (11)
about 30 minutes [2, pgll9]
Karlin proposes that the duration modifier is
only an approximation, and that it is the state
change modifier that determines the truth of the
sentence 1°. Most durations, however, in the do-
main of cooking tasks, are approximations. Decid-
ing whether a state change has been reached is also
8This in fact seems closer to logical conjunction than logical
disjunction.
9The exchanged order is usually used without the
or.
l°The terms left disjunct and right disjunct are used by Kar-
lin, but in sentences like (10) and (11) these are not helpful
indicators.
approximate. In a domain where durations and ev-
idence of state change are less approximate (say in
chemistry), it is not clear that it is always one of the
clauses that is performing the role of establishing
the truth of the sentence.
Aspectual category and verbal modifiers

Karlin's discussion is given in the context of the
aspectual category of an event (according to Moens
& Steedman [4]). This is useful as it gives a way of
extracting semantic information.
Karlin claims that points, culminations and cul-
minated processes (but not process type events) can
have a number of repetitions associated with them
(category 1). An expression whose aspectual type is
a process or culminated process can co-occur with a
duration modifier (category 2). This second claim
requires closer examination.
First, Moens & Steedman say that 'culminated
processes (do not combine readily) with a
for-
adverbial'. Yet for-adverbials are one of the classes
of duration modifier ennumerated by Karlin. We
look at two of the examples presented by Karlin.
Stir for I minute.
(12)
Saute over high heat until moisture is evaP-(13)
orated.
The expressions in both of these (without their
modifiers - that is
Stir and Saute over high heat)
are processes, but not culminated processes. An
essential part of a culmination is that there is a
consequent state [4, pg 16]. None of the exam-
ples Karlin uses has a culminated process as the
aspectual type of the action expressed. (13) could
be seen as culminated processes if viewed together

with the duration modifier (in other words, if it
already co-occurs with a duration modifier), while
(12) is a process, even with the modifier. Thus,
the
view of Moens & Steedman holds and is in fact use-
ful in extracting semantic information. An
until-
clause signals a culmination, thus making a process
into a culminated process. A
for-adverbial
does
not change the aspectual type of a process.
We now look at the assertion that 'Each process
in the cooking domain must have a culmination '
[3, pg 62]. This is accompanied by a claim that a
verb may contain inherent information about the
endpoint of the action it describes, as in
Chop the onion.
(14)
which, according to Karlin, describes a culminated
process whose endpoint is defined by the state of
the onion. This seems quite feasible, even if it
does require that some world knowledge is required.
However, if we consider instead the example
Stir the soup.
(15)
this does not describe any culmination, as there is
no consequent state. Yet it is a process, as it may
be extended in time.
281

Karlin's justification for the above assertion is
that cooking tasks involve a sequence of steps
with the goal of bringing about a state change.
There are also instructions for preventing a state
change though, for example stirring (to prevent
sticking). We could argue that stirring brings us
to the changed state
"stirred".
But then, if we look
back to the Moens & Steedman analysis, where he
climbed
has no culmination, we could claim that
this has the changed state
"has climbed".
This is
not the spirit intended by the Moens & Steedman
analysis, and it is more useful to see some actions in
cooking as not having culminations. We can then
examine what kinds of modifiers change aspectual
category and in what manner, as presented above
for the
for- and until-adverbials.
Conclusion
The semantics of repetition in instructions is more
clearly understood if we view repetition as having
scope and extent. Within this framework, Karlin's
work on the semantics of verbal modifiers provides a
useful starting point. In particular, relating this to
the aspectuai category of an instruction according
to Moens & Steedman [4] is important. We can

make use of Moens & Steedman's schema for the
way aspect changes when modifiers are added to
expressions, to extract semantic information. This
will allow a fuller treatment of extent, for use in
the development of a semantics for repetition that
treats both scope and extent more completely.
References
[1] Ball, C. N. "On the Interpretation of Descrip-
tive and Metaiinguistic Disjunction", unpub-
lished paper, University of Pennsylvania, Au-
gust 1985.
[2] Floyd, Keith
Floyd on Britain and Ireland,
BBC
Books, London,
1988.
[3] Karlin, Robin "Defining the semantics of ver-
bal modifiers in the domain of cooking tasks."
Proc. 26th Annual Meeting of the Association
for Computational Linguistics,
Buffalo NY,
USA, June 1988, pp. 61-67.
[4] Moens, Marc & Steedman, Mark "Temporal
Ontology and Temporal Reference."
Computa-
tional Linguistics
14:2, June i988, pp. 15-28
[5] Webber, Bonnie. Course description for In-
structions as Discourse, 3rd European Sum-
mer School in Language, Logic & Information,

Saarbrucken, August 1991.